Our calculation methods and sources

Calculation engine version 1, released November 12, 2026. Share links record this version so results can be traced if a formula ever changes.

Formulas

From the EDD we derive an equivalent LMP (EDD − 280 days) and estimated conception (EDD − 266 days). Gestational age on any date is 280 − (EDD − date) days, shown as completed weeks plus days.⁶

Inputs, units and limits

• All dates are calendar dates with no time of day. “Today” is your device’s local date.
• Cycle length: whole days from 20 to 45. Outside 24–38 days, the normal range in FIGO’s 2018 definitions, we show a note recommending scan dating.⁷ “Not sure or irregular” uses 28 days and says so.
• Scan age: 4+0 to 42+6. Weeks-pregnant input: 0+0 to 42+6.
• Crown-rump length: 6 to 84 mm, entered in millimetres. Above 84 mm, ACOG recommends dating by other measurements.¹
• Period, conception, transfer and scan dates can’t be in the future. A known due date can’t be more than 280 days away.

Rounding

CRL ages are rounded to the nearest whole day. For example, 45 mm gives 77.7 days, rounded to 11+1; 84 mm gives 97.5 days, rounded to 14+0. We also show the Hadlock (1992) result for the same CRL, computed with c in centimetres.^8,9 All other methods are exact whole-day arithmetic.

Derived dates

• Trimesters (ACOG): first through 13+6, second 14+0–27+6, third from 28+0.¹⁰
• Term categories (ACOG CO 579): early term 37+0–38+6, full term 39+0–40+6, late term 41+0–41+6, postterm from 42+0.¹¹
• Conception window: the six days ending on estimated conception, the interval in which intercourse led to pregnancy in Wilcox et al. (1995). It assumes ovulation on the estimated day, which often isn’t true.^12,13
• Week sizes: crown-rump length for weeks 7–13 from the inverse Robinson equation; estimated weight for weeks 14–40 from Hadlock’s 50th centile, ln(EFW) = 0.578 + 0.332·GA − 0.00354·GA² (GA in weeks), which reproduces the published 35 g at 10 weeks, 331 g at 20 and 3619 g at 40.¹⁴

Ultrasound redating thresholds

Source: ACOG Committee Opinion 700.¹ “More than” is strict: a difference equal to the threshold does not trigger redating.

Care windows

These are typical discussion windows, not appointments. Schedules vary by practice and by your health.

Reference cases

Each case is computed by the live engine when the site is built, and checked against an independently worked answer.

Testing

The engine is a standalone TypeScript module with an automated test suite. It checks every formula above, threshold equality, trimester and term boundaries, leap years, conversion against an independent calendar for every day from 1950 to 2100, daylight-saving transitions, and runs in several time zones. Dates are converted to whole day numbers, so no timestamp arithmetic is involved.

Limitations

• The calculator estimates dates. It cannot confirm a pregnancy, detect problems or predict the day of birth.
• Period-based dates assume ovulation timing that may be wrong, especially with irregular cycles.
• CRL conversion depends on a correct measurement, which needs a trained sonographer.
• Week-by-week sizes are population references, not measurements of any baby.
• The calculations and content have been checked against published sources, but have not been reviewed by a clinician.

Change log

• November 12, 2026, version 1. Launch: six dating methods, CRL conversion, ACOG redating comparison, US and UK care windows, calendar export.